1. Field of the Invention
The invention pertains generally to data transmission and reception, and, more particularly, to the reception of high speed data that has been used to modulate many carriers. This method of modulation is know by many names--Orthogonally Multiplexed Quadrature Amplitude Modulation (OMQAM), Dynamically Assigned Multiple QAM (DAMQAM), Orthogonal Frequency Division Multiplexing (OFDM); herein it is referred to as multicarrier modulation.
2. Prior Art
In general, oscillators (frequency sources) in the transmitting and receiving modems of a data communication link cannot by synchronized, and the receiving modem must use special circuitry or signal processing algorithms to adapt itself to the data rate and carrier frequency(ies) of the received signal; this task is often referred to as clock and carrier recovery.
The task is complicated by the introduction in some transmission media, most notably the General Switched Telephone Network (GSTN), of frequency offset and phase jitter as shown in FIG. 1. Frequency offset, designated herein as f, is the difference between the frequency(ies) of the received carrier(s) and the reference carrier(s) generated in the local receiver; it may be caused by (a) a frequency difference between the carrier(s) used in the transmitter and the reference carrier(s) in the receiver and/or (b) a mismatch between the frequencies of the modulating and demodulating carriers in Frequency Division Multiplexing (FDM) equipment in the network; on the GSTN the resultant combined offset may be as much as 5 Hz. Phase jitter is seen as phase modulation of the received signal, and often has a few discrete, identifiable frequency components; the power supply frequency and telephone ringing frequency (60 Hz and 20 Hz, respectively, in the U.S.A.) are common components. The carrier recovery circuitry or algorithms must track, or follow frequency offset and phase jitter in order to ameliorate their effect on the received signal.
Circuitry and algorithms for recovering clock and carrier (including tracking frequency offset and phase jitter) from data-modulated single-carrier signals have been well documented in the literature (see J. A. C. Bingham, The Theory and Practice of Modem Design. John Wiley, 1988). Nearly all of the prior art uses phase-locked loops (PLLs) of one form or another, and designers have long recognized the harmful effects of delay inside those loops. For carrier recovery it was agreed that tracking of phase jitter is made much more difficult, maybe even impossible, if the delay of an adaptive equalizer (typically about 10 ms) is included in the loop; therefore special algorithms (see, for example, D. D. Falconer, "Jointly Adaptive Equalization and Carrier Recovery in Two-Dimensional Communications Systems", Bell Syst. Tech J., vol. 55, pp. 317-334, March 1976) were developed to remove this delay from the loop.
In multicarrier modulation, data are grouped into blocks of bits; in the systems described by Baran (U.S. Pat. No. 4,438,511) and Hughes-Hartogs (U.S. Pat. No. 4,679,227) the blocks may comprise more than one thousand bits. Each carrier is modulated by just a few of those bits, and the modulation is held constant for the duration of one block; this duration, or symbol period, may therefore be several hundred times the symbol period of a single-carrier modem. Furthermore, the symbol period may be much greater than the periods of the components of the phase jitter that are to be tracked.
Signal processing in the multicarrier receiver must be performed at the symbol rate, and information about the received signal--data contained therein and imperfections (frequency offset, phase jitter, etc.) thereof--is available only after each block has been processed. This delay of one symbol period, which may be as large as 130 ms (more than ten times the delay through most adaptive equalizers), therefore appears inside the carrier recovery loop, and makes conventional jitter tracking impossible.
The long symbol period associated with multicarrier modulation makes the tracking of phase jitter imposed on multi-carrier signals a much more difficult problem than for single-carrier signals. One proposal (B. Hirosaki, et al, "A 19.2 kbit/s Voiceband Data Modem Based on Orthogonally Multiplexed QAM Techniques", IEEE Intl. Conf. Commun. Rec., pp. 661-665, August 1985) was to input an unmodulated pilot tone to a set (one for each carrier) of adaptive jitter predictors, and feed the output signals forward to cancel the jitter on each modulated carrier. This approach has several disadvantages: the amount of information about the jitter available from one pilot placed at the edge of the available frequency band is very small, the tapped-delay-line form of the predictors is poorly suited to filtering single-tones, and the approach requires a large amount of computation--particularly if a large number of carriers is used.
Another problem is that the jitter frequencies are not always known in advance--power supply and ringing frequencies vary from country to country, and other sources may generate significant components. Methods of identifying frequencies have been described, (e.g., the MUSIC algorithm described by R.O. Schmidt in "A Signal Subspace Approach to Multiple Emitter Location and Spectral Estimation", Ph.D. Thesis, Stanford University, 1981, and the ESPRIT algorithm described by R. Roy, A. Paulraj and T. Kailath in "ESPRIT--A Subspace Rotation Approach to Estimation of Parameters of Cisoids in Noise", IEEE Trans. ASSP, vol. ASSP-34, p. 1340-1342, October 1986) but they all require a very large amount of computation, and the particular problem of determining the frequencies of phase jitter has not been addressed.
Many of the early objections to multicarrier modulation were based on the assumption that correction for phase jitter was impossible because the problems of long symbol time and unknown jitter frequencies had not been, and probably could not be, satisfactorily solved.